1. Field of the Invention
The present invention relates to a semiconductor laser, and more particularly, to a distributed feedback semiconductor laser (hereinafter referred to as a DFB semiconductor laser) having a wavelength varying function.
2. Discussion of the Related Art
The DFB semiconductor laser is known as a device that has various applications in optical communication systems such as in optical cable television (CATV) technology, pumping light sources of an SHG short-wavelength laser for high-density information recording and of a small size solid-state laser, optical measurements, and the like. The conventional DFB semiconductor laser is commonly manufactured by a 2-stage (or more) epitaxial growth. Such DFB lasers are manufactured by forming a diffraction grating on a waveguide layer and then growing other layers epitaxially on the waveguide.
To avoid the complexity involved with an epitaxial growth of two or more stages, DFB semiconductor lasers have been developed that are manufactured having a 1-stage epitaxial growth. In recent years, the importance of providing such DFB semiconductor lasers with variable oscillation wavelengths has continued to increase.
Accordingly, DFB semiconductor lasers having variable oscillation wavelengths incorporating a chirped diffraction grating structure have been proposed. The chirped diffraction grating structure is classified into, for example, (a) a structure in which the diffraction grating of a DFB semiconductor laser is given a non-uniform pitch, and (b) a structure in which the diffraction grating of a DFB semiconductor laser is given a constant pitch and a bent waveguide is formed on a waveguide layer. The oscillation wavelength is varied in those manners.
However, method (a) requires a complex and difficult laser manufacturing process and hence it is not suitable for mass-production because a non-uniform diffraction grating needs to be formed. Method (b) results in a large loss of light because the light traveling direction is not perpendicular to the diffraction grating.